As an image pickup apparatus used for a medical image diagnosis based on radiation (X rays) or a nondestructive inspection, a radiation imaging apparatus using a flat panel detector (hereinafter, will be referred to as FPD) including a semiconductor element formed of a semiconductor material has been proposed. The above-described radiation imaging apparatus may be used as a digital image pickup apparatus for a still image, video, or the like in a medical image diagnosis, for example.
As the radiation imaging apparatus, a radiation imaging apparatus having an energy resolution has been proposed. For example, a photon counting type radiation imaging apparatus is configured to distinguish energy (wave length) of incident radiation and count the number of radiation detections in each of a plurality of energy levels. That is, a photon counting type sensor has an energy resolution and can be expected to be applied to discrimination of a substance, generation of an image in a case where shooting is performed by virtually using radiation of single energy, measurement of a density of bone, or the like. A detector including a conversion unit configured to convert radiation photons into optical photons or charges and a semiconductor element configured to obtain a pixel value in accordance with the optical photons or charges is used as the above-described radiation imaging apparatus having the energy resolution.
The radiation imaging apparatus having the energy resolution has an issue that the energy resolution is decreased by secondary radiation generated at the time of absorption of the radiation. PTL 1 describes a correction method of adding a value obtained by multiplying a measured value in a high energy band by a predetermined coefficient to the measured value in the high energy band and subtracting the value obtained by multiplying the measured value in the high energy band by the predetermined coefficient from a measured value in a low energy band. That is, while an error based on the secondary radiation having the lower energy than the incident radiation is taken into account, PTL 1 describes a technique for reducing the error based on the secondary radiation by adding a value obtained by multiplying the measured value in the high energy band obtained in accordance with the incident radiation by a predetermined coefficient corresponding to the error.